CA2022778A1

CA2022778A1 - Method for processing waste paper

Info

Publication number: CA2022778A1
Application number: CA 2022778
Authority: CA
Inventors: Wilfried Keck
Original assignee: Individual
Current assignee: JM Voith GmbH
Priority date: 1989-08-10
Filing date: 1990-08-07
Publication date: 1991-02-11
Also published as: CH681993A5; JPH0397984A; BR9003941A; FR2650768A1; NL9001790A; GB2234761A; FR2650768B1; DE4024561C2; GB2234761B; ES2024904A6; GB9014856D0; DE4024561A1

Abstract

ABSTRACT

A method and apparatus for processing waste paper has a vertical agitator with an accepts chamber and a conduit to convey the suspension to a storage tank. The conduit is screened prior to transfer to the storage tank and an impellor provided in front of the screen for cleaning and mixing. A proportion of the contents of the accepts chamber are recirculated through a separating device for removal of foreign objects.

Description

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The method relates to a method for processing waste paper.
A processing method is known from German Patent Specification 23 11 674, in which the inventive idea is to conserve energy by just slight slushing in the pulper and extensive slushing in the bleaching tower during a storage period of 2 to 3 hours. However, in this case additional apparatus in the form of a thickener is required in order to bring the suspension to the consistency of approximately 25~, with which operations are normally carried out in the bleaching tower. Therefore in this case there is a relatively low consistency in the pulper and a relatively high consistency in the bleaching tower. However this has the disadvantage that the pulper has a relatively low processing capacity in comparison with the bleaching tower.
The additional thickening apparatus, which involves high costs, is also disadvantageous. Furthermore, the slushing process in the pulper at the given consistency of approximately 5% to 6% is no longer economical nowadays and cannot be performed under the most technologically favourable conditions. It has to be borne in mind that impuritiss which are normally contained in waste paper are not extensively broken down, but are meant to be removed at an early stage.
As is known, nowadays pulpers are used with a relatively high consistency o~ 12% to 17~ and with a specific expenditure of energy of normally 25 Kwh/t and above (per tonne of waste paper) until there is a low residual flake content. For this purpose, there is used a slushing rotor, which has screw conveyors as the circulating components, which have an external diameter which continually decreases substantially towards the top, at least in its lower part, or which tapers conically at its outer circumference. As a result, the waste paper is gently broken up, predominantly by moderate shearing forces and 2~2~77~

fibre friction, i.e. the impurities are broken down as little as possiblQ by the action of the slushing rotor.
According to the present invention, there is provided a method for processing waste paper, wherein the waste papex is moderately broken up into mainly rags in a pulper at a consistency of between 8% and 12% with a maximum expenditure of energy of 15 Kwh/t, then the broken down waste paper rags are screened through a screening perforation of between 18mm and 25mm diameter, the screened waste paper rag suspension is conveyed by means of a pump into a storage tank and is stored there at a consistency of between 5% and 7% for the purpose of the further disintegration of the waste paper rags and flakes, and wherein the unscreened waste paper suspension is circulated from and to the pulper via an intermittently operating vortex flow appliance in a proportion of 6% to 20% of the pulper throughput, whereby the waste paper rags in the suspension are further broken up and the fibre content therein undergoes further screening so as to return to the pulper only waste paper constituents which can be slushed.
According also to the present invention, there is provided apparatus for processing waste paper comprising a pulper, a storage tank for the accept from the pulper and a vortex flow and screening appliance providing a circulation path from and to the pulper, wherein a rotor is disposed on the floor of the pulper and having a vertical axis of rotation, wherein said rotor has conveying screws having an external diameter which gradually decreases at least in the lower part of the rotor in the upward direction, or having an external diameter which decreases conically, wherein an accept chamber is housed on one side of the pulper, which is separated by a substantially flat screen which deviates from the vertical by max. 15 r has perforations of between 18mm and 25mm, and wherein an impeller is mounted for rotation in front of the wire to keep the latter clean and for mixing.

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It has been found in practice that there is no additional deflaking at the slushing rotor or slushing wheel of said rotor, i.e. by the rotating blades of said rotor brushing past a screenO The result is an even more gentle separation of the waste paper, in which the impurities can be gradually removed from the pulper in a particularly fa~ourable way through the circulation. After the paper rags have been separated, the accept is screened out in the circulating equipment and returned to the pulper again.
The specific slushing energy in the pulper is kept low and is at most 10 Kwh/t to 15 Kwh/t waste paper, depending upon the strength or wet strength of said paper;
in practice, slushing is only performed up to pumping capability, in which case there is a flake content of bstween 30% and 40% related to the whole (paper) fibre content.
A delivery chamber having a screen separating it from the rest of the pulper, parallel to which and along which the arms or blades of a further rotor similar to an impeller can be moved to keep it clear, is not provided in the direct vicinity of the slushing rotor, but on one side wall of the pulper.
The invention is explained below by means of an exemplified embodiment shown in the f igures of the drawings where Figure 1 shows schematically a waste paper processing apparatus partially in section; and Figure 2 shows a horizontal section through the apparatus of Figure 1.
The waste paper is supplied to the tank of a pulper 1 in bales 15 or loose via a conveyor 14. Here it is separated into shreds of rags by the action of screw conveyors 3 of a conveying rotor 2, which becomes continually narrower in the upward direction at its circumference/ at least in its lower part, or becomes ~27~

narrower in a conical shape as shown in Figure 1. In this case, the consistency in the pulper is between 8% and 12%, preferably 10% and 12%. The slushing rotor has a vertical axis of rotation and is disposed on the floor of the pulper and is driven by a motor 31 via a drive shaft 30. On one side of the tank wall of the pulper, there is disposed a screen 7 having substantially uniform perforations of 18mm to 25mm diameter, which separates an accept chamber 8 from the rest of the tank. In front of the screen an impeller 5 is located and rotates to keep the screen clear and to mix the contents of the pulper in this region to a certain extent by means of diluting medium supplied through line 13.
On the pulper there is an outlet 6 from which a line 26 leads to a vortex flow appliance 10. The appliance 10 has an impeller 11 which rotates to draw suspension ~rom the pulper at a rate oE 6% to 20% of its throughput and feed this via line 16 to a screening appliance in the form of a perforated screening drum 17. This drum is housed in bearings 19 on its shaft 18, for example any non-utilizable dirt is discarded through apertures at the front on the left-hand side of the screening appliance. These apertures are defined between radial spokes 20, by means of which the drum is attached to its shaft 18. Of course it would also be possible to support the screening drum on rollers over its circumferences.
The necessary diluting medium is supplied to the circulation, i.e. via a line 34 into the line 26a.
Heavy material is removed form the pulper as normal via heavy particle sluice 27.
From Figure 2 it can be seen that the accept chamber 8 and the screen 7 are disposed on a projection 28 of the pulper which is more or less on one side. As a result the suspension does not flow tangentially past the impeller 5, but is supplied at a steep angle thereto and to the screen. This improves the throughput of the accept.

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The screen 8 is substantially vertical or at a maximum is inclined at 15 to the vertical.
In the accept chamber 8, the waste paper suspension achieves a consistency of approximately 5% to 7%
and this suspension is then pumped to a tower-shaped storage tank 22 via a line 26 and pump 29. There it remains for roughly 1 to 2 hours, at most 2 to 3 hours, for further (chemical) slushing and it is then conveyed through a line 25 for a subsequent processing operation. Whilst in the tank 22 the suspension can optionally be further diluted via a pipe 24 with the help of propeller 23 to roughly 4% before the subsequent processing operation.
of course, further screening stages are provided below, in which waste paper bales which have not yet broken up are partially broken up.
The storage tank 22 is preferably constructed so that it is naturally aspirated whereby pumping can be carried out even with non-avoidable larger amounts of t:rapped air. It has been shown that this is perfectly possible.
Consequently in contrast to systems with slushing consistencies of 12% to 17% in the pulper, which are also frequently used nowadays, a strong dilution associated with subsequent thickeniny and thus a thickener in the "front' part of the process is avoided.
It can also be seen from Figure 2 that the angle "a" between the radial of the pulper drawn through the cutting line of the central axis of impellor 5 and screen 7 with~ the wire surface is roughly an angle of 120. This angle may preferably be between 115 and 130. The angle of the ~3ide wall of the pulper in this upstream region with the said radial may be between 20 and 50, i.e. it may be up to 5 smaller and up to 10 larger than the angle between the shaft of the impeller and the radial.

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However it must be remembered that the slushing rotor 2 does not rotate above a screen and is not close to the screen as is customary, with the result that it does not exert any considerahle tearing and deflaking action and it consequently breaks up the waste paper in a particularly gentle wayO The intermittent circulation of the fibre suspension by means of vortex flow appliance 10 via line 16 and screening device 17 is known from USP 4,634,059.
The vortex flow appliance may be a pump working on the vortex principle (see USP 4,370,172 or Papier, Carton et Cellulose 1986, page 57, Figure 5) or an appliance as specified in USP 4,634,059, in which the ratio of the (axial) length to the diameter of the interior of the housing may lie between 0.8 and 1.5. The motor 31 is intermittently driven at between 10 and 120 cycles per hour (i.e. 6 minutes to 30 seconds) with the high values applying more to the vortex pump.
The slushing chemicals, i.e. 1.5 - 4 % NaOH or Na2SO3 or a part thereof, may preferably already have been added to the pulper, just like any chemicals required for subsequent flotation.
The slushing in the pulper results in a relatively high proportion of residual paper rages, but these can to a large extent be screened through the large wire perforations.

Claims

1. A method for processing waste paper, wherein the waste paper is moderately broken up into mainly rags in a pulper at a consistency of between 8% and 12% with a maximum expenditure of energy of 15 Kwh/t, then the broken down waste paper rags are screened through a screening perforation of between 18mm and 25mm diameter, the screened waste paper rag suspension is conveyed by means of a pump into a storage tank and is stored there at a consistency of between 5% and 7% for the purpose of the further disintegration of the waste paper rags and flakes, and wherein the unscreened waste paper suspension is circulated from and to the pulper via an intermittently operating vortex flow appliance in a proportion of 6% to 20% of the pulper throughput, whereby the waste paper rags in the suspension are further broken up and the fibre content therein undergoes further screening so as to return to the pulper only was paper constituents which can he slushed.

2. A method according to claim 1 characterized in that the slushing process is performed in the pulper under the action of moderate shearing forces produced by a rotor with helical blades.

3. A method according to claim 1 or 2 characterized in the the waste paper is broken up in the pulper at a consistency of between 10% and 12% and is stored in the tower-shaped tank for 1 to 2 hours.

4. A method according to any one of claims 1 or 2 characterized in that heavy particles are removed separately from the pulper via a heavy particle sluice.

5. A method according to any one of clams 1 or 2 characterized in that screening is performed in a region of the pulper remote from the slushing rotor.

6. A method according to claim 1 or 2 characterized in that heavy particles are removed separately from the pulper via a heavy particle sluice and the screening is performed in a region of the pulper remote from the slushing rotor.

7. A method according to any one of claims 1 or 2 characterized in that slushing is performed in the pulper substantially right up to the pumping capability of the waste paper suspension.

8. A method according to any one of claims 1 or 2 characterized in that slushing is performed in the pulper for a maximum period of 6 minutes.

9. A method according to any one of claims 1 to 7 characterized in that chemical slushing is performed in the storage tank for a transit time of 1 to 2 hours.

10. Apparatus for performing a method according to any preceding claim comprising a pulper, a storage tank for the accept from the pulper and a vortex flow and screening appliance providing a circulation path from and to the pulper, wherein a rotor is disposed on the floor of the pulper and having a vertical axis of rotation, wherein said rotor has conveying screws having an external diameter which gradually decreases at least in the lower part of the rotor in the upward direction, or having an external diameter which decreases conically, wherein an accept chamber is housed on one side of the pulper, which is separated by a substantially flat screen which deviates from the vertical by not more than 15°, and has perforations of between 18mm and 25mm, and wherein an impeller is mounted for rotation in front of the wire to keep the latter clean and for mixing.

11. Apparatus according to claim 10 characterized in that the cross-section of the pulper corresponds at least partially to circular sections and in that the screen is disposed in an extension piece projecting in the radially outward direction, so that inside the large part of the pulper in the horizontal cross-section of the pulper, the wire surface forms an angle of between 115° and 130° with the radial of the pulper through the intersection point of the central axis of the impeller with the screen surface in the upstream region of the circulating flow of the suspension the cross-sectional plane of the pulper, and the lateral wall of the pulper in this region forms an angle between 20° and 50° with the said radial.

12. Apparatus according to claim 10 or 11 characterized in that the vortex flow appliance is a pump according to the vortex system.

13. Apparatus according to claim 10 or 11 characterized in that the vortex flow appliance has a cylindrical housing with a rotor similar to an impeller disposed on one axial end and with the inlet aperture in the centre of the outer axial end, with the cross-section of the interior of the housing in the axial section having a ratio of between 0.8 and 1.5 to 1 for the ratio of the length to the diameter.

14. Apparatus according to any one of claims 10 or 11 characterized in that the screening device is a substantially horizontal, perforated drum which can rotate around its central axis.